What can I do to enjoy being single while my friends aren't?

is your hobby running? and instagraming?

Go out there and find new friends if you don't feel comfortable

Well I know your feeling. All of my friends are married, and I'm not, so when I'm with 'em I feel like a 3rd wheel. I guess hang out with us online. Or just go out and find some new friends to hang out with.

I suppose casual sex isn't an attractive option...Well, if it's not, there's always the endless search for "the one". Eff that though. Search for fun. You only get so many spins on this rock. Enjoy the hell out of em.

If you still want to hang out with friends without being second choice then you could ask to hang with them and their boyfriends.
But as for what to do... is your hobby a social one where you can meet new friends? If not, then just get out there and enjoy life and hopefully you'll make new single friends while doing so. But I wouldn't completely discount your coupled up friends - one day they may well be single too and you'll be best buds again. You're not second best right now, they're probably just in the honeymoon phase of their relationship where they want to be with the guy 24/7.

Do you know any other who is single if so maybe you should hangout with them, that way you don't feel odd and lonely. Do something that you love to do such as go out shopping, read a book or magazines, walk to a lake feed ducks, take photos of things that makes you happy, scrap booking, I don't know find something that you enjoy and keeps you staying happy while being single.

The only thing I could suggest is finding more friends who are single as well ? is there a club or group you can join at your school or somewhere around where you live?
what about old friends you haven't talked to in awhile ? try to reconnect with them.
Or you can always go out by yourself and do things that you enjoy - shopping, museum, spa day, photography. Start a new hobby?